Background noise is often superimposed on a voice signal inputted from a microphone, and a hand set or the like, and it becomes a big problem when voice coding or speech recognition is performed. As a signal processing device which aimed at cancelling a noise superimposed acoustically, a two-input type noise cancellation device using two adaptive filters is disclosed in patent literatures 1 and 2. Using a signal to noise ratio in a primary signal estimated by using the first adaptive filter among two adaptive filters, a stepsize calculation unit calculates a coefficient-update stepsize of the second adaptive filter. The first adaptive filter operates like the second adaptive filter, but a value larger than the coefficient-update stepsize of the second adaptive filter is set to a coefficient-update stepsize of the first adaptive filter. For this reason, although the first adaptive filter has a high follow-up performance of output to an environmental variation, its estimation accuracy of the noise is inferior to the second adaptive filter.
The stepsize calculation unit, based on the signal to noise ratio in the primary signal estimated by using the first adaptive filter, assumes that interference by a voice signal is large when the voice signal is larger than the noise, and provides a small coefficient-update stepsize to the second adaptive filter. On the contrary, the step size calculation unit assumes that interference by the voice signal is small when the voice signal is smaller than the noise, and provides a large coefficient-update stepsize to the second adaptive filter. Thus, by controlling the second adaptive filter with the coefficient-update step size provided by the stepsize calculation unit, a noise-cancelled signal which accomplishes sufficient follow-up performance to the environmental variation and low distortion in the signal after noise cancellation, simultaneously.
Further, in patent literature 3, a structure of a noise cancellation device which, by extending a structure of patent literatures 1 and 2 mentioned above, also performs cancellation of the voice signal mixed in the background noise, when an influence of the voice signal mixed in a background noise is large, that is, a so-called crosstalk of the voice signal exists, is disclosed. In patent literature 3, in addition to the structure of patent literatures 1 and 2 mentioned above, a second stepsize calculation unit calculates the coefficient-update stepsize of the adaptive filter which cancels the voice signal from the noise input signal in order to cancel the background noise correctly from the voice signal input.